1999A&A...351.1087L

A CO survey of most of the southwest half of M 31 at an effective angular resolution of 1' has been performed with the 14-m radiotelescope of the Five College Radio Astronomy Observatory QUARRY receiver array. At the 200pc linear resolution of this instrument at the distance of M 31 (690kpc), the CO emission, already known to peak in a broad Population I ring 10kpc in radius, is found to be concentrated in two spiral arm segments, Baade's (1963, Evolution of stars amd galaxies. Harvard University Press, Cambridge) S3 and S4, both well defined in space and velocity. Fainter CO emission features are also detected further from the center, mainly in the outer arm S5, and closer to the center. The prominent molecular arms S3 and S4 are aligned with dust and atomic counterparts. At the intersection of S4 with the major axis, there is an offset between the arm defined by gas and dust and that defined by the classical tracers of Population I objects. The HII regions are mostly concentrated along the outer edge of the gaseous arm, and the OB associations are found even further out. However, no significant streaming motions are detected across the arm. Although overall M 31 is several times fainter in CO than the Milky Way, the CO distributions in the two galaxies are similar in certain respects. The prominent molecular spiral arm S4 in M 31 is very much like the Carina arm in our Galaxy at the same galactocentric radius, and beyond a radius of about 8kpc (the solar circle), M 31 and the Milky Way are very similar in their molecular content. The overall difference in CO intensity between the two galaxies originates in the inner 7kpc where M 31 is very dim and the Milky Way very bright. However, even in this inner region, common features are found. First, the ratio between the CO intensity and the HI column density changes in the same way with galactocentric radius in both systems, rising inwards to about 2-3kpc from the center. Second, the molecular gas in the inner two kiloparsecs appears in both cases to follow a perturbed kinematics, perhaps as a result of a stellar bar.